Hi Suzanne,
All the records that are assigned to this user will remain assigned to him if you make him inactive and remove his ability to sign in. The user will not actually be deleted from Method, just marked as inactive (therefore not showing up in your users list unless you click Show inactive users). If you like, you could change this user to someone else by changing the user name, password, email address, etc. and then all of the records previously assigned to the old user will now be assigned to the new user.
Hope this makes sense!
Jason